This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have expressed and purified multiple soluble, clade A and B HIV-1 glycoproteins as gp140 trimers, soluble D1D2 CD4 and a panel of neutralizing anti-HIV antibodies. While these reagents are used in quantitative binding studies and ongoing crystallization trials, SAXS can be used to obtain complementary structural information that can independently have significant impact on our studies of the biophysical basis of the process of neutralization by antibodies. We specifically propose to use SAXS to determine the overall conformation of gp140 trimers (which will be compared to cryo-EM structures of viral Env spikes and crystal structures of gp120 and gp41) and the location of binding (and any induced gross conformational changes) of our panel of NAbs and sCD4. Clade A &B gp140s, and gp140 constructs with specific regions deleted (V1), will also be used in comparisons to glean details of the architecture of these molecules.